In one case, they put together ads for houses up for sale or rent in North Carolina.

They found that ads for houses on sale were delivered to an audience that was 75% white users. Ads for houses for rent were shown to a more mixed demographic.

They also ran identical ads for houses while varying the image: in one case an advert contained a white family, in the other a black family. The ad when run with the white family was served to 85% white users, while the same ad with the black family was served to an audience of 73% white users. This is despite the fact the researchers set the ads to be targeted identically.

The researchers also ran job ads, and found that the kind of job they were advertising affected who was targeted. Ads for jobs in the lumber industry reached users who were 72% white and 90% men. Supermarket cashier jobs were delivered at to an audience of 85% women, while jobs with taxi companies went to a 75% black audience.

The issue at the heart of the findings is not that Facebook is deliberately skewing ad results along racial and gender lines, but rather that its ad-targeting systems could be doing so automatically.

"We stand against discrimination in any form. We've announced important changes to our ad targeting tools and know that this is only a first step. We've been looking at our ad delivery system and have engaged industry leaders, academics, and civil rights experts on this very topic — and we're exploring more changes."